1. Field of the Invention
The present invention relates generally to a technique for detecting a recurring signal contained in an information signal stream, and more particularly to a technique for detecting periodic synchronization signals that frame data packets in a synchronous data signal stream.
2. Description of the Known Art
Telecommunication systems enable information to be communicated from one point to another, by modulating the information in analog or digital form on a radio frequency (RF) signal. The RF signal is then radiated from an antenna having sufficient gain and directivity to ensure that the RF signal can be detected above background noise at the receiving site. Cellular radio telephone systems operate to communicate speech information in analog or digitized form between one of a number of base stations, and a subscriber unit installed in the subscriber's vehicle or carried by the subscriber. Currently, there is a trend toward upgrading existing analog cellular telephone systems by enabling them to handle digitized speech conversations with the added benefit of increased system capacity.
Unlike analog systems, the transmission of speech in a digital communications system requires the insertion of synchronization (sync) flags or signals at known, fixed intervals in a speech data signal stream to allow the information to be decoded with least error at the receiver site. The use of digital communications technology in a cellular radio telephone network enables multiple subscribers to converse over the network simultaneously on a single frequency channel, using, e.g., a time division multiple access (TDMA) scheme. With TDMA, each subscriber has access to the cellular network during an assigned time slot within a data signal frame. When a particular user's time slot is available, the user's speech data may be encoded and transmitted at a greater rate than that needed if the data were allocated over an entire data signal frame.
Because digital speech data originating simultaneously from three or more different subscribers may be transmitted in corresponding time slots of each signal frame, a predetermined sync pulse or burst is used to mark the beginning of each frame. The sync pulses are detected by speech data processors in the network which rely on the timing of the pulses to ensure that the speech data in each of the frame time slots is identified with the proper subscriber. Each sync pulse has a known interval or duration (.tau.) and repeats in the speech data stream with a known periodicity (T). For example, in a so-called MIRS digital cellular radio telephone system, the frame sync signal recurs every 15 msec and the pulse duration is 0.75 msec. Sync pulses or signals are modulated or encoded on a system RF carrier signal in a given manner. For example, in digital communications systems, sync pulses may be chosen to be phase (PSK), amplitude (ASK), or frequency (FSK) modulated, or both phase and amplitude modulated (QAM).
Certain peripheral equipment made for use with digital communications systems, including but not limited to digital cellular systems, must be able to detect the system sync pulses or signals so that the equipment can operate properly within the system. Such equipment may require input clock or timing signals that are in phase with the system sync signals. The clock signals may be used by the equipment to ensure, for example, that downlink synchronous signals transmitted from a base station are properly routed to a number of mobile receivers.
It may not always be possible to connect or couple the peripheral equipment directly to the system sync signal source, however. Even if a continuous sample of the transmitted data signal stream is obtained, it has still been necessary to derive correlation coefficients by using complex and finely tuned circuit networks, in order to determine the location and timing of the sync pulses in the signal stream.